Image forming apparatus are known which form an electrostatic latent image on the surface of an image carrying body such as a photoreceptor drum that is charged uniformly in advance by, for example, exposing the image carrying body surface to light, develop the electrostatic latent image by sticking a developer such as charged toner on it, and transfer the developer to a recording subject medium such as a recording sheet. As for image forming apparatus of this type, it is pointed out that in the case where a non-magnetic, one-component toner is used as the developer in such a manner as to be friction-charged, the toner deteriorates and its charging characteristic varies as image formation is performed repeatedly. In view of this, in image forming apparatus of this type, it has been proposed to change the development bias in accordance with the number of printed sheets, as disclosed in JP-A-2003-173074